1. Field of the Invention
The present invention relates to an encoder sensor and an image forming apparatus including the encoder sensor.
2. Description of the Related Art
A printer, a facsimile, a copier, a plotter, or a multifunction machine including functions of a printer, a facsimile, a copier, and a plotter are known examples of an image forming apparatus. The image forming apparatus may be a liquid jet recording type image forming apparatus using a recording head that ejects ink droplets. The liquid jet recording type image forming apparatus performs image forming (also referred to as “recording”, “character printing”, “image printing”, “printing”) by ejecting ink droplets from a recording head to a conveyed sheet of paper. The liquid jet recording type image forming apparatus includes a serial type image forming apparatus and a line type image forming apparatus. The serial type image forming apparatus performs image forming by ejecting ink droplets from a recording head while moving the recording head in a main scanning direction. The line type image forming apparatus performs image forming by ejecting ink droplets from a recording head without moving the recording head.
It is to be noted that, in the below-described embodiments of the present invention, the term “image forming apparatus” refers to an apparatus that performs image forming by, ejecting ink droplets onto a medium such as paper material, thread material, fiber material, cloth material, leather material, metal material, plastic material, glass material, wood material, and/or ceramic material. Further, the term “image forming” not only refers to ejecting ink droplets onto a medium for forming an image having significance such as a character or a figure but also refers to simply allowing ink droplets to drop onto a medium for forming an image having no particular significance such as a pattern. Further, the term “ink” not only refers to ink but also refers to liquids that can be used for image forming such as a recording liquid, a fixing solution, or a resin liquid. Further, the term “sheet of paper” not only refers to a sheet of paper material but also refers to a sheet of a material to which droplets of ink are applied (e.g., OHP sheet, sheet of cloth). The sheet of paper may also be referred to as a target recording medium, a recording medium, or recording paper. Further, the term “image” not only refers to a flat two-dimensionally formed image but also refers to a three-dimensionally formed image.
Further, it is to be noted that an image forming part included in a liquid jet type image forming apparatus according to the below-described embodiments of the present invention is not limited to a liquid jet.
One example of the liquid jet type image forming apparatus is the serial type image forming apparatus. The serial type image forming apparatus includes a linear encoder (position detection apparatus) having an encoder scale and an encoder sensor. The encoder scale is positioned along a main scanning direction of a carriage on which a liquid jet head is mounted. The encoder sensor is for reading patterns (position distinguishing parts) of the encoder scale. Thereby, the linear encoder detects the position and speed of the carriage and controls, for example, the speed of the carriage or the driving of the liquid jet head based on the detection results. Another example of the liquid jet type image forming apparatus includes a rotary encoder (position detection apparatus) having a wheel-like encoder scale (also referred to as “encoder wheel”) and an encoder sensor. The wheel-like encoder scale is provided to a conveying part (e.g., a roller, belt) conveying a target recording medium on which an image is formed by an image forming part. The encoder sensor is for reading patterns of the wheel-like encoder scale. Thereby, the rotary encoder detects the position and speed of the conveying part and controls, for example, the driving of the conveying part based on the detection results.
There are various types of conventional linear encoders such as a magnetic type linear encoder or an optical type linear encoder. For example, the magnetic type linear encoder has an advantage of having its performance hardly affected by a small amount of stain on a surface of the linear scale. However, the magnetic type linear encoder has disadvantages such as difficulty in attaining precise resolution, difficulty in increasing the gap between the linear scale and the encoder sensor, difficulty in achieving precise attachment, and difficulty of handling magnetic tools. On the other hand, the optical type linear encoder, for example, has advantages such as being relatively easy to increase the gap between the encoder scale and the encoder sensor, being easy to assemble, and being suitable for attaining precise resolution.
Due to increasing resolution of the linear encoder along with the increasing speed and precision of the image forming apparatus, liquid and paper particles or the like become scattered inside the image forming apparatus. This leads to problems such as output being degraded and signals being erroneously output. For example, in a case where the image forming apparatus is used for a long period, ink mist and paper particles adhere to the encoder scale and the encoder sensor. This leads to readout errors. Such readout errors cause deviation of the position of the carriage that result in disarrangement of recorded images and generation of errors that result in shutdown of the image forming apparatus.
In light of the above, there is proposed an inkjet recording apparatus including an encoder sensor provided with plural LEDs of different colors for utilizing an aspect that the wavelength of absorbable light is different depending on the color of ink mist (see, for example, Japanese Laid-Open Patent Publication No. 2007-55050 (Patent Document 1)). This inkjet recording apparatus anticipates the color of ink mist adhered to the encoder scale based on the value obtained by counting ejected ink droplets and switches the color of light irradiated from the LED for preventing light from being absorbed.
Further, there is proposed an image forming apparatus including a cleaning member attached to a lower side of an encoder sensor for cleaning the surface of an encoder scale (see, for example, Japanese Laid-Open Patent Publication No. 2008-179103 (Patent Document 2)). This image forming apparatus also includes a gap position changing part which raises/lowers a carriage between a cleaning position (position where the cleaning member contacts the surface of the encoder scale) and a withdrawing position (position where the cleaning member does not contact the surface of the encoder scale). Accordingly, the image forming apparatus uses the cleaning member to clean the stains adhered to the surface of the encoder scale by moving the carriage in the main scanning direction to a state where the cleaning member is in the cleaning position.
However, with the inkjet recording apparatus of Patent Document 1, there may be a case where there is no difference in the wavelengths of absorbable light depending on the characteristics of the ink mist adhered to the encoder scale. In such a case, light cannot be prevented from being absorbed. Further, although the inkjet recording apparatus of Patent Document 1 may prevent the performance of the encoder sensor from degrading in a case where a large amount of a single color ink is used, degradation of the performance of the encoder sensor cannot be prevented in a case where ink of all colors are uniformly used. Further, the inkjet recording apparatus of Patent Document 1 cannot handle a problem where a substance other than ink mist adheres to the encoder sensor.
With the image forming apparatus of Patent Document 2, it is difficult to completely remove stains from the surface of the encoder scale by using the cleaning member. Further, the image forming apparatus of Patent Document 2 cannot sufficiently prevent detection precision from degrading.